1. Field of the Invention
The present invention relates to a technology, which estimates an exhaust gas temperature of one of an upstream side and a downstream side of an exhaust gas purifying device based on an exhaust gas temperature of the other one of the upstream side and the downstream side, in a control system for an internal combustion, which includes the exhaust gas purifying device.
2. Description of Related Art
For example, in a diesel engine, there is provided a diesel particulate filter (DPF) as an exhaust gas purifying device for capturing particulate matters (PM) in exhaust gas. Because the captured PM is accumulated in the DPF, a DPF regeneration control is regularly performed in order to burn and remove the PM. When a temperature of the DPF rises too high in the DPF regeneration control, the DPF may disadvantageously deteriorate. Therefore, an exhaust gas temperature sensor is provided on an upstream side of the DPF, and in the DPF regeneration control, a combustion state in the DPF is adjusted based on the exhaust gas temperature on the upstream side of the DPF sensed by the exhaust gas temperature sensor.
An air-fuel ratio sensor, which senses an oxygen concentration (air-fuel ratio) in the exhaust gas, is provided on the downstream side of the DPF. The air-fuel ratio sensor includes, for example, a sensor element made of a solid electrolyte of zirconia, and the sensor element is kept at a predetermined activation temperature (e.g., 750° C.) such that a temperature detection signal, which corresponds to the oxygen concentration, is outputted. The sensor element internally has a heater, and when the heater generates heat by energization, the sensor element is heated such that the activation state can be maintained.
The oxygen concentration detected by the above air-fuel ratio sensor and a energization amount for energizing the heater of the air-fuel ratio sensor are influenced by exhaust gas heat of therearound to vary. Thus, an exhaust gas temperature sensor is provided on the downstream side of the DPF (e.g., near the air-fuel ratio sensor), and the influence by the exhaust gas heat is compensated based on the exhaust gas temperature on the downstream side of the DPF sensed by the exhaust gas temperature sensor. Also, when an engine is started in cold (i.e., when a cold start is made), moisture in the exhaust gas may be condensed such that condensed water may be generated. Then, when the condensed water is applied to the sensor element heated by the heater, the sensor element may be disadvantageously broken. Therefore, it is determined whether or not there is condensed water in an exhaust pipe (i.e., a wet-and-dry state is determined) based on the exhaust gas temperature on the downstream side of the DPF sensed by the exhaust gas temperature sensor. In accordance with the determination result, it is determined whether the heater is energized or not.
As described above, it is indispensable to know the exhaust gas temperature on the upstream side and the downstream side of the DPF in order to protect the DPF in the DPF regeneration control, and also in order to improve degree of detection accuracy of the air-fuel ratio sensor. In other words, exhaust gas temperature sensors need to be provided on the upstream side and the downstream side of the DPF, respectively, to sense corresponding exhaust gas temperatures. However, from a view point of cost reduction, the number of the exhaust gas temperature sensors needs to be reduced.
In order to solve the above disadvantages, there is disclosed a technology, where an exhaust gas temperature, which is sensed by an exhaust gas temperature sensor provided on one of the upstream side and the downstream side the DPF, is used to estimate an exhaust gas temperature of the other one of the upstream side and the downstream side. For example, in JP-A-2005-140069, a temperature change in the DPF is modeled by a transfer function, which is expressed by “first-order lag+dead time”, and the temperature is estimated based on the model. In JP-A-2005-245109, similarly to JP-A-2005-140069, a transfer function, which is expressed by “n-th order lag+dead time” is used to model a temperature change in the DPF. However, each of these transfer functions is expressed by an approximate expression, which is experimentally computed, and therefore, a degree of estimation accuracy may not be disadvantageously sufficient.